Field of the Invention
The invention concerns a method for current measurement, especially in gas-insulated voltage switchgear, for example, in a high or medium-voltage unit. It also pertains to a current measurement configuration for execution of the method.
A so-called measurement transformer or measurement sensor is ordinarily used as a current transformer to measure currents in such gas-insulated switchgear, in which a gas referred to as SF6 is generally used as an insulating gas. The measurement transformer essentially includes, as an information sensor, a coil that concentrically surrounds a current conductor of the unit that conducts a primary current. A secondary current flowing in the coil is then referred to as measurement signal, which is processed in a subassembly of an automation system also referred to as process control technique for the switchgear (LSA).
As described in the essay xe2x80x9c20 Years of Research in the Field of Nonconventional Transformersxe2x80x9d, in ELEKTRIE, Berlin 43 (1989) 6, pages 205 to 207, two different transformer principles can be distinguished. In a so-called conventional transformer, the information sensor and a power supply connected to it are coupled on the primary side to the current conductor of the unit and connected together to the actual load on the secondary side. However, the transformer requires large installation volume. On the other hand, in a so-called non-conventional transformer only the information sensor is coupled to the current conductor of the unit on the primary side. An auxiliary power supply is therefore required in the non-conventional transformer, which is capable of producing a power of up to a few kW, at least for a short time, to maintain the desired nominal output currents of the secondary circuit of 5 or 1. A power amplifier having a corresponding parameter, via which the information sensor is connected to the load, is therefore required. An optical solution is particularly suited for this type of transformer, but can only be implemented at a very high cost.
An intermediate solution is therefore often sought, which underlies the ordinary or conventional measurement principle, but at a low-power level. A low-power measurement transformer based on the intermediate solution, however, is particularly complicated to manufacture, especially if a large saturation-free time is required.
It is also known from Published, European Patent Application 0 510 311 A2, corresponding to U.S. Pat. No. 5,272,460, that a current transformer that operates according to the principle of a Rogowski coil can be used for a metal-encapsulated, gas-insulated high-voltage unit. A current measurement configuration according to the principle of the Rogowski coil is also known from Published, Non-Prosecuted German Patent Application DE 44 24 368 A1. To achieve high imaging trueness in the measurement range while using a Rogowski coil, high expenditure, with correspondingly high costs, is required. The reason for this is that the measurement accuracy of the Rogowski coil, on the one hand, is determined by a number of factors, and that special precautions are required, on the other hand, for high reliability and constancy of parameters, owing to the system-related off-line configuration.
It is accordingly an object of the invention to provide a method and a configuration for current measurement that overcome the above-mentioned disadvantages of the prior art methods and devices of this general type.
With the foregoing and other objects in view there is provided, in accordance with the invention, a method for measuring current, which includes: comparing a first signal generated by a first low-power measurement transformer having a magnetic core and a second signal generated by a second nonmagnetic measurement transformer; selecting and further processing one of the first signal and the second signal in dependence on an imaging accuracy of a measured current; and evaluating a deviation between the first signal and the second signal as comparison criterion.
The underlying task of the invention is therefore to provide a particularly suited method and a particularly suited configuration for precise measurement of currents with high dynamic range.
The task is solved with reference to the method by comparing a first signal of a first low-power measurement transformer with a magnetic core and a second signal of a second nonmagnetic, especially optical, measurement transformer. Depending on the accuracy of imaging of the measured current, one of the two signals is then selected and further processed. A deviation of the two signals emitted by the measurement transformers of different type from each other is evaluated as comparison criterion.
The invention starts from the consideration that accurate measurement of currents with high dynamic range can be achieved at low cost and limited dimensions by using a combined current measurement element. Only a sensor or transformer is to be operated in the actual measurement range, whereas another sensor is used, essentially in the overcurrent range. The imaging trueness or accuracy in the measurement range can be guaranteed by a low-powered measurement transformer, whereas a Rogowski coil or an optical measurement transformer, operating according to the Faraday effect, takes over imaging of high, stationary or displaced currents. Rotation of the plane of polarization of linearly polarized light occurs, according to the Faraday effect in an optically inactive material in the presence of a magnetic field parallel to the direction of light propagation.
The signals recorded separately by the two measurement transformers, i.e., their output voltages, are expediently processed together. The output voltage of the low-power measurement transformer drops in the event of its saturation. The gradient, for example, the first or second derivative, of its output voltage is then greater than the gradient of the output voltage of the measurement transformer operating according to the principle of the Rogowski coil or according to the Faraday effect. The derivative of output voltages, according to time, can therefore be referred to as a gauge of the accuracy of the two signals. The gradient of the output voltages or output signals of the two measurement transformers is therefore expediently considered as criterion for switching the output signal of the low-power measurement transformer to the output signal of the Rogowski coil or the optical measurement transformer. As an alternative or in addition, a check and evaluation of characteristic features of the two output signals can occur by a fuzzy controller and/or a neuronal net, to form a switching criterion and thus to determine the degree of accuracy.
With the foregoing and other objects in view there is provided, in accordance with the invention, a configuration for measuring current, including: a first low-power measurement transformer having a magnetic core and recording a primary current conducted in a conductor traversed by current and converting the primary current into a first secondary current; a second nonmagnetic measurement transformer for recording the primary current conducted in the conductor traversed by the current and converting the primary current into a second secondary current; a processing component; an input component connected to and downstream from the first low-power measurement transformer for converting the first secondary current of the first low-power measurement transformer to a first output signal and feeding the first output signal to the processing component; a series circuit connected to and downstream of the second nonmagnetic measurement transformer and including an integrator component and a signal amplifier for converting the second secondary current of the second nonmagnetic measurement transformer into a second output signal and feeding the second output signal to the processing component; and the processing component switching from processing the first output signal to the second output signal in dependence on a comparison of curve shapes of the first output signal and the second output signal in the processing component if the magnetic core of the first low-power measurement transformer reaches saturation.
The current measurement configuration includes a combined current measurement element with a low-power measurement transformer and a powerless or optical measurement transformer, especially a Rogowski coil. An input subassembly for level adjustment and potential separation is expediently connected after the low-power measurement transformer. The input subassembly expediently includes, as signal amplifier, an operating amplifier, as well as a voltage-limiting component in the form of a bipolar Zener diode as a protective circuit to guarantee electromagnetic compatibility (EMC protection).
On the other hand, a passive preintegrator is expediently connected after the second measurement transformer, i.e., especially the Rogowski coil, to which an amplifier is connected (again via an EMC protection in the form of a bipolar Zener diode). This can also be a buffer amplifier.
Both measurement transformers are connected to a common processing component for signal processing via the subassemblies connected downstream. The processing component or processing subassembly includes one analog/digital converter (A/D converter) each for the signals produced by the measurement transformers. During use of only a single A/D converter for both signals together, a multiplexer is connected upstream. Evaluation and further processing of the signals transmitted by the measurement transformers expediently occurs on the digital side.
The advantages achieved with the invention consist especially of the fact that both high imaging trueness in the measurement range and good transmission behavior in displaced currents in the overcurrent range and thus a high saturation-free time are achieved by using a combined current measurement element with a low-power measurement transformer and with a Rogowski coil. The expenditure and thus the costs are particularly limited during the use of this type of combined current measurement element, in comparison with a conventional measurement transformer with a connected A/D converter. The configuration is also characterized by high reliability and particularly limited incorporation volume in gas-insulated switchgear. Moreover, direct connection to electronic protection devices is possible without problem.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a method and a configuration for current measurement, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.